newbie question about repos

[Morten Gerdsen]

Hi,

I've recently installed FC5 for the first time, so I'm sorry if this is
a stupid question: In my Yum repo list I can see a lot of repos that are
not being used by my Yum, such as "core-source", "extras-development",
"legacy testing" etc etc.

Am I right in assuming that some of these are beta-versions of new
packages, and would it be a mistake to use them? 

I realize that some might be unstable, but couldn't I just uninstall the
new versions and reinstall the old ones if something was too buggy?

I guess I am asking: which repos should I use if I want to stay
"bleeding edge", and which one should I avoid under all circumstances?

thanks!
Morten